GB,
are you mixing your spray mixture with pond water. if so try it with clean tap water, also use a good surfactant.
Sonar or Avast will eradicate duckweed as well as all the other aquatic plants in the pond. just simply pour it in the pond in a few places, it will do the rest. the only drawback to these chemicals is that they are very slow acting, it takes around 90 days to see results. so if the pond has a creek feeding it, the herbicide will probably be washed out before it has a chance to work. If your pond has no creek coming in and a small amount of water flow then Sonar or Avast will work great.
Also if you have flowing water you can keep adding Sonar/Avast monthly to keep the floridone concentration high but its very costly.
